TN295 



No. 8964 








































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8964 



Bureau of Mines Information Circular/1984 



Ground Penetrating Radar 

A Review of Its Application 
in the Mining Industry 



By W. E. Pittman, Jr., R. H. Church, 
W. E. Webb, and J. T. McLendon 




UNITED STATES DEPARTMENT OF THE INTERIOR 



((iPnMMiU. ^AJUtMA^fh^i^ 



Information Circular/8964 



Ground Penetrating Radar 

A Review of Its Application 
in the Mining Industry 



By W. E. Pittman, Jr., R. H. Church, 
W. E. Webb, and J. T. McLendon 




UNITED STATES DEPARTMENT OF THE INTERIOR 
William P. Clark, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 

Research at the Tuscaloosa Research Center is carried out under a memorandum of agreement between 
the Bureau of Mines, U.S. Department of the Interior, and the University of Alabama. 




Library of Congress Cataloging in Publication Data: 



T 



^0 






Ground penetrating radar. 

(Information circular / United States Department of the Interior, Bu- 
reau of Mines ; 8964) 

Bibliography: p. 16-23. 

Supt. of Docs, no.: I 28.27:8964. 

1. Mine safety— Equipment and supplies. 2. Ground penetrating ra- 
dar. I. Pittman, Walter E. II, Series: Information circular (United 
States. Bureau of Mines) ; 8964. 



TN295.U4 622s [622'.8] 83-600319 



^6° 






CONTENTS 

\ji Page 

(V 

^ Abstract 1 

\ Introduction 1 

~- Origins of GFR concepts 2 

"" Electromagnetic transmission through the earth 3 

Electrical characteristics of earth and rock « 4 

Electronic characteristics of coal 5 

Applications of GFR 6 

Radar measurements of Ice and snow thickness 6 

Radar measurements of salt domes 7 

Ar cheologlcal and geotechnlcal applications 7 

Subhlghway measurements 8 

Microwave measurements of coal seams 8 

GPR systems 9 

Video pulse radar applied to GPR 9 

Short pulse GPR 10 

Borehole radar 10 

Synthetic pulse radar 11 

Strata control radar 12 

GPR antennas 14 

Interpretation of GPR data 14 

Conclus Ions 15 

Bibliography 16 

ILLUSTRATIONS 

1 . Block diagram of a simple short pulse radar system 2 

2 . Block diagram of a synthetic pulse radar system 3 

3. Radar control console of GPR used during underground testing 13 

4. Antenna deployment against mine roof 14 



to 



o 
-2 





UNIT OF MEASURE 


ABBREVIATIONS USED IN 


THIS REPORT 


A 


ampere 




lb/in2 


pound per square inch 


cm 


centimeter 




m 


meter 


dB 


decibel 




MHz 


megahertz 


dB/ft 


decibel per 


foot 


mi/h 


mile per hour 


op 


degree Fahrenheit 


mm 


millimeter 


ft 


feet 




m/s 


meter per second 


GHz 


gigahertz 




MW 


megawatt 


Hz 


hertz 




ns 


nanosecond 


in 


inch 




pet 


percent 


kHz 


kilohertz 




V 


volt 


km 


kilometer 




W 


watt 


kW 


kilowatt 









GROUND PENETRATING RADAR 

A Review of Its Application in the Mining Industry 

By W. E. Pittman, Jr./ R. H. Church, ^ W. E. Webb,^ 
and J. T. McLendon 



ABSTRACT 

The Bureau of Mines, as part of its Health and Safety Technology Pro- 
gram, is conducting research on the use of ground penetrating radar 
(GPR) for mine hazard detection. GPR offers a possibility of mapping 
immediate below-surf ace geologic conditions, including faulting and 
other anomalies, thereby exposing potential safety hazards to miners. 
This report summarizes a literature review of the current status of GPR 
research in and outside the Bureau. 

INTRODUCTION 

The presence of unsuspected geologic anomalies in coal-bearing strata 
poses a threat to the safety of coal miners during mining. Clay veins 
can create structural weaknesses and cause the collapse of mine open- 
ings. Uncharted well casings, sand channels, sulfur balls, "horse- 
backs," "kettlebottoms ," and mine voids are known to be hazardous 
features. While coal mining will remain a dangerous industry, it is 
obvious that advance knowledge of what lies around the mine opening and 
ahead of the coal face could save many lives and also improve the effi- 
ciency of coal production. Many techniques have been proposed for 
premining investigation and some have proved valuable. GPR is one of 
those holding promise of becoming an important method of detecting 
near-face hazards. 

As part of its continuing effort to advance mine safety technology, 
the Bureau of Mines has been conducting research on various types of 
GPR's, for use from the surface, from the underground working face, or 
from a borehole. This report reviews the historical progress of GPR's 
and the current status of research to apply them in the mining 
industry, 

^Technical information specialist, Tuscaloosa Research Center, Bureau of Mines, 
Tuscaloosa, Ala.; professor of the history of science, Mississippi University for 
Women, Columbus, Miss. 

^Mining engineer, Tuscaloosa Research Center. 

-^Physical scientist, Tuscaloosa Research Center; professor of electrical engineer- 
ing. University of Alabama, Tuscaloosa, Ala, 



ORIGINS OF GPR CONCEPTS 



GPR probably owes most of its begin- 
nings to military research. In particu- 
lar, the U.S. Army has maintained a long 
term interest in GPR as a rapid means of 
detecting land mines and subsurface tun- 
nels. In 1956, Cook separately proposed 
and demonstrated the use of an airborne 
radar to measure the thickness of float- 
ing sea ice (33-34) .^ He recognized that 
there were two interfaces (air-ice, ice- 
seawater) , each of which would give clear 
radar returns. The thickness of the ice 
could be determined from the different 
times of arrival at the receiver of the 
two echoes and knowledge of the electro- 
magnetic wave's transit speed in ice. 
Frequencies between 75 and 200 MHz were 
used to achieve penetration of the ice, 
using a single cycle or "pulse" of the 
radiofrequency (RF) carrier for good dis- 
tance resolution of the two echoes. This 
has been the primary concept adopted for 
use in most GPR's. 



delay, phase, polarization, and propaga- 
tion direction. The return signals are 
normally displayed and recorded as ampli- 
tude with respect to delay time. The 
frequencies used are usually in the 30- 
MHz to 2-GHz range (8^, 23). 

Another form of GPR is the video pulse 
radar. It transmits a very short pulse 
with a bandwidth that is very wide. This 
video pulse output spectrum spreads from 
essentially direct current to beyond 3 
GHz. The signal scattered by the target 
can then be sampled over a very broad 
frequency range, and information about 
the target can be derived from each sam- 
ple. The extremely short pulse also al- 
lows for "time isolation" ; the trans- 
mitted pulse magnitude can fall to a very 
small value before the reflected pulse 
returns. This avoids signal interference 
(23, 25). 



However, several variations of the sin- 
gle or short pulse radar systems have 
been proposed and tested for use in GPR. 
Most of the successful ones have been of 
the short pulse type (fig. 1). In GPR 
work, compared with standard radar use, 
the distances involved are very short. 
Hence, the times of travel of an electro- 
magnetic radar pulse and its reflection 
from a target are correspondingly short. 
To avoid interference between outgoing 
pulses and the returning reflected 
pulses, it is necessary to keep the 
pulses extremely short. The problem is 
complicated by the fact that certain fre- 
quencies can achieve better depth pene- 
tration, and these optimum frequencies 
vary according to the nature and type of 
the soil or rock. 

The most common type of short pulse GPR 
uses a short pulse of known waveform 
whose return signal is received in the 
time domain and can provide information 
about the target from its amplitude, time 

^Underlined numbers in parentheses re- 
fer to items in the bibliography at the 
end of this report. 



Transmitter 



1 



r 



Receiver 



Transmit 
antenna 



Transmit 
electronics 



Receive 
antenna 



Receive 
electronics 



_l 



Controller 



Timing 
control 



J 



"I 



Amplifiers 



Filters 






r 



Tape 
recorder 



Facsimile 
display 



FIGURE 1. - Block diagram of a simple short 
pulse radar system. 



Synthetic pulse radar (fig. 2) trans- 
mits a single frequency at a time. The 
returning signal is measured for its am- 
plitude and phase, and the transmitter 
frequency is then stepped up to the next 
value, and so on. From the return data, 
a pulse can be reconstructed by taking 
the inverse Fourier transform of the re- 
ceived signal ( 146 ) . 

A frequency modulated-continuous wave 
(FM-CW) GPR system sweeps through a band 
of frequencies that it transmits and 
then compares the instantaneous output 



frequency with the frequency of the sig- 
nal reflected from the target. The dif- 
ference in frequencies is a function of 
the distance to the target and the di- 
electric constant (or constants) of the 
material in the wave path (8, 46-49). 

Each of these GPR systems has advan- 
tages and disadvantages. Each system al- 
so involves the use of digital circuitry 
to analyze the returning signal from the 
target. Workable models of each system 
have been developed and tested as de- 
scribed later in the text. 



ELECTROMAGNETIC TRANSMISSION THROUGH THE EARTH 



Studies on the transmission of electro- 
magnetic waves through the earth date 
back to the 1920' s. These were undertak- 
en for geophysical research and to devel- 
op through-the-earth mine communications , 
and many of the studies were done by the 
Bureau of Mines (72-74, 76 , 123 ). By the 
1960's, theoretical models of electromag- 
netic wave propagation in the earth had 



been developed, owing largely to the work 
of James R. Wait and others. 

In a series of seminal studies ( 129- 
137 ) , Wait analyzed the theoretical 
response of the uniform earth to vari- 
ous forms of electromagnetic excita- 
tion and derived mathematical solutions 
to describe it. He, along with F. C. 



Tape 
drive 



Control unit 



Control 
micro- 
processor 



A/D 
conversion 



Synthe- 
sizer 



^ 



F.O. trans 



Offset 
synthesizer 



I MHz if 
reference 



Sample 
and bold 



Sample 
and hold 



Transmitter 



I__j 1^ Power _ 

jV — V amplifier 

I F.O. X' ' 

cable ^ F.O. RCVR. 



Quadrature 
hybrid 



mixer 



I r 



90' 



mixer 



U ! 



ANT 



XMIT-if 
reference 



Receiver 



1st mixer 



Amplifier 




ANT 



FIGURE 2. - Block diagram of a synthetic pulse radar system. 



Frischknecht and others, later extended 
the analyses to a two (or more) layered 
earth (59, 127-128 , 137 ). These cases 
utilized detectable interfaces within the 
earth. Investigators in the 1950' s and 
1960's also studied cases involving vary- 
ing frequencies, as well as transmissions 
from loops, dipoles, and infinitely long 
wires in various locations and orienta- 
tions (23-J^, 60, 62, 125-126 ). Much of 
this theoretical work, which underlies 
most through-the-earth electromagnetic 
radiation studies, would later prove to 
have only limited applicability to GPR 
(27, 61, 64, 67, 80, 111-113 ). The fre- 
quencies studied were so much lower than 
those used in actual GPR that different 
electronic phenomena predominated. 

A coal seam presented a special case. 
Wait pointed out that electromagnetic 
waves could propagate laterally in re- 
sistive layers ( 138 ) . A coal seam can be 
considered a resistive slab bound by con- 
ductive rock. In such a model, the domi- 
nant mode of transmission of electro- 
magnetic energy has low attenuation, a 
result that many observers had already 
noted empirically. 

Not all RF energy penetrated deeply 
into the earth. Clough (29) pointed out 
that an electromagnetic wave impinging 
upon the earth at the critical angle 
given by Snell's law will be refracted 
and will give rise to lateral waves. 
These will usually be of low amplitude 
and lost in other signals, but they could 
predominate in some modes. This might be 
particularly true of GPR in contact with 
the ground surface or a coal face. The 
physical basis of electromagnetic surface 
waves was studied by Lytle, Miller, and 
Lager (^9^), who described the physical 
phenomena that occurred and derived the 
mathematics relating them. 

A method of determining the distance to 
a buried target was described by Clay, 
Greischor, and Kan (28^) in 1974. Using 
matched filter detection, a technique al- 
ready common in other electronic applica- 
tions, the investigators were able to de- 
termine the distance to conducting layers 



in the earth. In this method, a set of 
filters are constructed to match the 
waveforms of signals that have traveled 
to different depths within the earth and 
have been reflected. 

ELECTRICAL CHARACTERISTICS 
OF EARTH AND ROCK 

Concurrently, there was much research 
being done to determine the basic elec- 
trical characteristics of soil and rock. 
The mechanisms by which solids propagate 
electromagnetic energy were discussed at 
length by Von Hippel ( 124 ) and Meakins 
(90) . Collett and Katsube also reviewed 
electrical characteristics of rock (con- 
ductivity and permittivity, or alternate- 
ly, loss tangent), how they are deter- 
mined, and the measuring systems and 
techniques used to determine these char- 
acteristics in the frequency range 10^ to 
10^ Hz (30). Ward and Eraser ( 142 ) also 
described the conduction of electricity 
in rocks. Methods of measuring rock 
electrical characteristics were reported 
by Yatsyshin, Zhuk, Salamatov, and Yako- 
vitskaya ( 147 ) and Lytle (85) , who in- 
cluded a good overview of the subject as 
well, and Khalafalla and Viner (78) , who 
described methods of dielectrometry ap- 
plicable to the 10^- to 10^-Hz range. 

There are many other valuable published 
reports (6^, 41, 66^, 79, 103 , 118 , 122 ). 
They vary as to the type of soil or rock 
studied, frequencies used, method of mea- 
surement, whether the measurements were 
in situ or in the laboratory, and other 
factors. Watt, Mathews, and Maxwell pub- 
lished an early study of Earth crust 
electrical characteristics ( 144 ) , while 
Griffin and Marovelli determined the di- 
electric constants and attenuation fac- 
tors for six basic rock types in the 
frequency range 20 to 100 MHz (63) . 
Hoekstra and Delaney (70) studied the di- 
electric constants of soils at ultrahigh 
frequency (UHF) and microwave frequency 
ranges. Saint-Amant and Strangway also 
reported on the dielectric constants of 
various rocks and soils ( 1 10 ) in a de- 
tailed investigation of powdered and sol- 
id dry rocks in the frequency range 50 



kHz to 2 MHz. They found that both the 
dielectric constant and the loss tangent 
increase with increasing frequency and 
increasing temperature. At high enough 
frequency (over 1 MHz), the loss tangent 
approaches a constant value. A valuable 
study by Hipp ( 68 ) related electromagnet- 
ic propagation parameters to functions of 
frequency, soil density, and soil mois- 
ture. The last has particular applica- 
bility to coal. In a related study, 
Campbell and Ulrichs ( 21 ) pointed out the 
geologic implications that might be de- 
rived from the electrical characteristics 
of lunar rocks. 

ELECTRONIC CHARACTERISTICS OF COAL 

Other studies have a more direct bear- 
ing upon GPR (2, 4-, 50). Cook, in 1970, 
reported his investigations of bituminous 
coal, in which he tried to determine the 
transparency of coal to very high fre- 
quency (VHF) radiation (32). Two fresh 
bituminous coal samples were tested at 1, 
5, 25, and 100 MHz in a measuring circuit 
utilizing a capacitance test cell and an 
RF bridge. In situ tests were also car- 
ried out. He found that at 60-dB signal 
attenuation he could obtain a 50-m pene- 
tration with a 100-MHz signal and a 1- to 
10-km penetration distance with a 1-MHz 
signal. The presence of clay or pyrite 
veins in the signal path strongly af- 
fected the penetration and prevented du- 
plication of these results. Cook then 
undertook the systematic study of a rep- 
resentative range of rocks (38 kinds) to 
determine their transparency to radar 
frequency electromagnetic energy (35) . 
Using short, broadband pulses at 1, 5, 
25, and 100 MHz, he measured the trans- 
mission penetration with a circuit using 
an RF impedance bridge and a parallel- 
plate capacitance test cell. He found 
that low-loss propagation was possible 
in certain granites, limestones, coals, 
and dry concrete, and that (then) exist- 
ing VHF mining radar could explore effec- 
tively for distances of up to several 



hundreds of feet. Shorter but still use- 
ful distances were possible for other 
coals , gypsums , oil shales , dry sand- 
stone, high-grade tar sands, and schists. 
For most shales, clays, and fine-grained 
soils, probing distances were generally 
restricted to less than 10 ft. Signifi- 
cantly, RF losses increased by as much as 
a factor of 12 when the samples were 
wetted. The controlling factor determin- 
ing rock transparency to radar evidently 
was the uncombined moisture content of 
the rock. 

Balanis, Jeffrey, and Yoon (_5) measured 
the microwave transmission characteris- 
tics of coal samples. Using two coal 
samples, one fine and one granular, each 
at two "typical" moisture levels (10 and 
15 pet) , they measured the transmission 
of 8.2- to 12.4-GHz waves through the 
samples. Two different waveguide tech- 
niques were used in the tests, to measure 
a sample with finite length as well as 
to simulate a sample of infinite length. 
Later, Balanis, Shephard, Ting, and Kar- 
dosh (_7) reported on the anisotropic 
properties of coal in the radar frequency 
ranges, by using a two-path interferom- 
eter at 9 GHz. They measured the dielec- 
tric constant and conductivity of coal as 
a function of the direction of propaga- 
tion of the transient signal and as a 
function of the polarization of the elec- 
tromagnetic wave in the coal. Using four 
Eastern bituminous coals and one Eastern 
anthracite coal, they also studied the 
effects of certain other physical proper- 
ties upon high frequency electromagnetic 
conduction. These included the rank of 
the coal, the pyrite concentration and 
distribution, the mineral content, and 
the moisture content. Anthracites have 
higher conductivities by a factor of 10, 
and higher permittivities by a factor of 
2 to 3 , than do bituminous coals. The 
investigators also found that high fre- 
quency electromagnetic anisotropy corre- 
lated quite well with optical anisotropy. 



APPLICATIONS OF GPR 



RADAR MEASUREMENTS OF ICE 
AND SNOW THICKNESS 

While basic work on earth dielectrics 
and conductivity continued, there was al- 
so further research ( 101 ) into the prac- 
tical applications of GPR. An earlier 
proposal of Cook had been the use of air- 
borne VHF pulse radar for the measurement 
of ground ice and snow (33) . The idea 
soon became reality in Antarctica, where 
intensive multinational explorations were 
in progress. Airborne VHF pulse radar 
in various vehicles proved an invaluable 
tool in profiling the snow and ice masses 
of the continent (52-53, 55, 106 ). The 
methods were quickly applied elsewhere. 
Luchinonov (84) reported on the use in 
the Soviet Union of airborne radar to 
delineate mountain glaciers. In Canada, 
airborne radar was used to measure the 
depths of ice and snow ( 99 , 102 ) . Camp- 
bell and Orange (20) reported on the use 
of pulse radar for the continuous pro- 
filing of sea and fresh water ice thick- 
ness in Arctic regions. GPR was also 
shown ( 88 ) to be useful in permafrost 
as it penetrated deeply enough to be 
meaningful. 

Annan (_3) reported in 1976 on the use 
of a wideband (150 MHz), short pulse ra- 
dar with a 110-MHz frequency, 50-W out- 
put, and a 50-kHz repetition rate for 
underground surveying in the Arctic. The 
long wavelengths of the radar limited the 
resolution to features near the surface. 
Higher frequencies were suggested for fu- 
ture use. Despite its inadequacies, GPR 
in general proved its utility as an un- 
derground surveying method, particularly 
if used in conjunction with drilling. 
Wide-angle reflection and refraction 
sounding techniques (WARR) , used in seis- 
mic sounding, were suggested for use in 
layered areas. Reliable sounding from 3 
to 30 m should then to be possible with 
GPR in permafrost. In a related experi- 
ment, Bertram, Campbell, and Sandler (11) 
reported using short pulse radar to lo- 
cate large masses of underground ice. 



Harrison and a British group ( 65 ) used 
a pulse-modulated radar at 35 MHz from a 
U.S. Navy aircraft to penetrate glacial 
ice and allow the creation of a graphical 
reconstruction of the underlying terrain. 
The radar succeeded in delineating the 
various interfaces between ice and rock, 
ice and air, brine and ice, brine and 
air, and so on. The radar signal returns 
were recorded on 35-mm film for later 
analysis, which proved difficult because 
of the complicated ray paths of the 
radar signals and echoes. The relative- 
ly long wavelengths used did not give 
great resolution, but the system provided 
a valuable tool for subglacial explora- 
tion through maximum ice depths of 1 to 
3,000 m. 

On the frozen Great Lakes, Cooper, Mul- 
ler, and Schertler (40) experimented with 
the use of a GPR mounted on an all- 
terrain vehicle for rapid profiling of 
the lake ice depth. Earlier experiments 
from a C-47 aircraft at a 2,300-m alti- 
tude and a 75-m/s groundspeed had given 
inconclusive results because of the in- 
herent difficulties in locating the air- 
craft's position accurately enough to 
make the later calibration measurements 
by surface auger meaningful. They could 
not determine how close to reality the 
observed thicknesses (10 to 92 cm) actu- 
ally were. 

Experiments with a Coast Guard heli- 
copter showed similar disadvantages. 
The airborne radar did not react to 
ice thicknesses of less than 10 cm, and 
any surface melt water over 1 mm thick 
blocked the radar. Hoping to overcome 
these problems, a short pulse, 2.86-GHz 
radar with 20-W peak power was mounted on 
an all-terrain vehicle to be used on the 
frozen lake surface. 

Extensive laboratory measurements of 
the dielectric constant of lake ice were 
made under various conditions before the 
actual tests were made on the lake. As 
the vehicle-borne radar proceeded across 



the lake ice, work teams bored auger 
holes for verification measurements. The 
results showed a maximum depth overesti- 
mate by radar of 9.8 pet, a maximum un- 
derestimate of 6.6 pet, and an average 
error of 0.1 pet. Upon analysis, it ap- 
peared that most of the Inaccuracy was 
due to error in auger depth measurements 
and the rest to nonuniform dielectric 
constants or the resolution of radar at 
the wavelength used. 

RADAR MEASUREMENTS OF SALT DOMES 

GPR's have also been used in salt domes 
where the much lower RF attenuation of 
salt has allowed its use over much great- 
er penetration ranges. Several research- 
ers associated with Texas A, & M, Univer- 
sity (69^, 116 , 122 ) did early work in 
this area. One group ( 71 ) utilized bore- 
hole radar, made small enough and rugged 
enough to be used in an oil well under 
10,000-lb/in2 pressure and at 250° F. 
The 250-MHz pulse radar with a 400-Hz 
pulse-repetition rate and a 10-kW peak 
power was lowered into the borehole. At- 
tenuation through salt ranged from 1.2 to 
2.8 dB per 100 ft, which gave penetration 
ranges through rock and salt of up to 
8,208 ft. Internal reflecting surfaces 
within the salt body produced anomalous 
reflections that were difficult to inter- 
pret. Despite this and other difficul- 
ties of interpretation created by an in- 
sufficient data base, the technique was 
considered a viable tool for geophysical 
exploration. Another group ( 115 ) made 
extensive tests of a 440-MHz radar in 
the Cote Blanche Salt Dome. It was used 
within the mine to outline the salt body 
and to detect discontinuities within the 
salt. The roof of the salt dome was also 
outlined. Penetration ranges of up to 
2,040 ft were achieved at low power, and 
3,177-ft ranges were achieved at high 
power. 

ARCHEOLOGICAL AND GEOTECHNICAL 
APPLICATIONS 

GPR has been used in archeology as an 
exploration tool by a group ( 43 ) seeking 
a means of detecting hidden chambers 
within the Great Pyramids of Egypt. The 



"sounder" operated in the frequency range 
16 to 50 MHz, with a pulse 1-1/2 cycles 
in length and a peak power of 0.2 MW. It 
was tested in a Western dolomite mine 
where empty chambers 100 to 130 ft from 
the transmitter produced good radar 
echoes. Lytle and Lager (86) used a 3- 
to 50-MHz transmitter at the Wowona tun- 
nel in Yosemite National Park for tests 
from within the tunnel to the surface, 
through 300 m of solid granite. They 
measured the in situ bulk conductivity of 
the rock as well as the relative dielec- 
tric constant as a function of frequency. 

Geophysical Survey System, Inc. , engi- 
neers (99) , have put together a small 
portable array of GPR's designed for 
near-surface location of various targets. 
The signal is a wideband (120 MHz) pulse 
with a repetition rate of 50 kHz. The 
antenna is a small wheeled carriage that 
can be towed or pulled by hand up to 2-3 
mi/h. The radar receiver's output is by 
means of a graphic recorder, a continuous 
strip chart that provides profiles of the 
underground target. The set is easily 
portable even in an experimental form. 
It has low power requirements (12 V, 1.5 
A) and uses the same antenna for trans- 
mission and reception. The system uses 
a time-domain sampling technique, which 
reduces the signal frequency to audio 
ranges where it can be conveniently re- 
corded on the strip chart. The unit has 
been successfully tested for the detec- 
tion of pipes buried up to 13 ft below 
the surface, for use in the presence of 
multiple targets, to detect underground 
wires, and to profile lake and delta 
deposits. 

Another imaginative use of GPR came 
from New Mexico where National Bureau of 
Standards (NBS) investigators, working 
with the National Park Service (91) , used 
a FM-CW radar to find the structural 
weaknesses of an ancient Spanish mission 
undergoing restoration. The technique 
holds much promise for archeological 
work, particularly since the radar re- 
turns give an accurate representation of 
the water distribution in the sample 
area. 



GPR was also suggested (10) as a basic 
tool for geotechnical site assessment in 
conjunction with traditional survey meth- 
ods. Radar was suggested as a cheap and 
effective method to fill in between ex- 
pensive exploratory boreholes. It also 
could be used in hydrological and soil 
studies, and analyses of hazardous mate- 
rial disposal sites. However, GPR ap- 
pears to be extremely "site specific" in 
its ability to penetrate the earth be- 
cause of the differences in soil and the 
presence or absence of water. The U.S. 
Air Force (12) has also shown interest in 
GPR for use in rapid evaluation of air- 
field pavements for damage under combat 
conditions. 

SUBHIGHWAY MEASUREMENTS 

There is a growing awareness of the po- 
tential for subsurface testing offered by 
GPR. It is obvious GPR is a viable new 
tool for geologists and engineers ( 106 ) . 
Highways continue to be a fruitful area 
for the application of the new technol- 
ogy. A persistent problem for highway 
builders is the instability of sloping 
ground, caused by subsurface geologic 
anomalies. Exploratory drilling is ex- 
pensive and frequently gives incomplete 
data. Working with the Federal Highway 
Administration (FHWA) and the U.S. Geo- 
logical Survey (USGS) , NBS developed four 
microwave probing systems ( 120 ) . From 
them, the FM-CW system was chosen for 
highways investigation because of its 
suitable resolution at ranges of inter- 
est. It had adequate penetration, could 
be built with commercially available com- 
ponents , and had previously been success- 
ful in a subsurface survey. The FM-CW 
system was built and subjected to 5 weeks 
of testing in the Pike National Forest in 
Colorado. Boreholes and borehole TV cam- 
eras were used to determine the details 
of underground geology at the test site 
and to verify the results of the radar 
tests. The FM-CW radar successfully lo- 
cated a series of fractures in granite at 
depths between 5.96 and 6.86 m. 



Moore, Echard, and Neill (98) reported 
the use of a short pulse radar (approxi- 
mately 1 ns) to detect the presence of 
voids under concrete slabs on an inter- 
state highway. Experiments were done on 
test blocks and on a highway. The vary- 
ing radar responses seemed to delineate a 
true picture of the underslab voids, but 
it was impossible to verify them. When 
the group attempted to drill into the 
voids for verification, the cooling water 
used in the drills rushed through the 
newly opened holes into the voids , carry- 
ing with it dirt and concrete, disturbing 
the voids. Also, the effect of the dif- 
ferences in dielectric constant of the 
concrete and earth was poorly understood 
and was not compensated for properly. 
The technique works , but a large body of 
experimental data will have to be built 
up for it to reach full potential. 

MICROWAVE MEASUREMENTS OF COAL SEAMS 

Delineation of coal seams (thickness, 
shape, and orientation) using microwave 
radar was a subject of several investiga- 
tions. A theoretical basis for the new 
technology was derived by Wait ( 139 ) . 
Under a Bureau of Mines contract, NBS 
(47) used a microwave (0.5 to 4.0 GHz) 
radar to measure coal seam thickness in 
two coal mines. The technique worked to 
measure coal thicknesses from 10 to 40 
cm, but some anomalies in the coal seams 
confused the results. However, the re- 
searchers recognized that detection of 
these anomalies might be the most valu- 
able characteristic of microwave sensing. 
They also found that the dielectric con- 
stant was a function of the water 
content. 

NBS subsequently developed a FM-CW ra- 
dar system operating in the 1 to 2 GHz 
range (48-49) . It was tested in the lab- 
oratory and in three coal mines to mea- 
sure coal seam thickness up to 55 cm. 
The technique was suggested for possible 
use in controlling the operation of auto- 
mated mining machinery at the coal face. 



GPR SYSTEMS 



VIDEO PULSE RADAR APPLIED TO GPR 

Video pulse radar techniques have been 
applied to GPR. A video pulse radar uses 
broadband signaling but with the wave- 
forms consisting of multiple frequencies 
(from near direct current to megahertz or 
gigahertz), instead of broadcasting many 
cycles of one frequency, as does a tradi- 
tional radar. This multif requency wave 
will be perturbed by any change in under- 
ground electrical characteristics such as 
different soil or rock types that have 
differing dielectric constants, or in- 
terfaces such as cavities, joints, or 
faults. The transmitter sweeps through 
the frequencies, and the reflected elec- 
tromagnetic wave is picked up by the re- 
ceiver to give a voltage-versus-time out- 
put. The waveforms can be processed for 
both time and frequency domains. This is 
done experimentally by a digital comput- 
er, but in actual operating units a 
built-in logic circuitry would do it. 
Using a video pulse radar, Moffatt ( 94 ) 
easily detected a tunnel through 20 ft of 
limestone, as well as some previously un- 
suspected steel supports. An effort to 
detect faulting in a quarry produced am- 
biguous results, however. The complex 
fracturing of the rock made it difficult 
to relate the electronic information to 
physical reality. 

A study by Burrell and Peters ( 17 ) 
pointed out that using the "Low Frequency 
Window" (LFW) , a frequency range in which 
the attenuation of RF waves is relatively 
low (below 100 kHz), penetration ranges 
of several kilometers were possible. One 
problem was the need to decouple the pow- 
erful transmitted signal from the weak 
reflected wave, which it could mask. So 
that the return signal could be observed, 
it was necessary for the output signal to 
fall to zero very rapidly. This was ac- 
complished by including a high frequency 
portion of the pulse and by developing 
orthogonal antennas to electronically 
separate the output and input signals. 

Video pulse radar was used by Cook 
in a series of experiments (37) for the 



British National Coal Board. The British 
needed to detect unmapped, abandoned 
mineworkings in advance of the mining 
face, because abandoned galleries, if 
penetrated unexpectedly, could release 
floods of mud or water under high pres- 
sure. Cook and his associates reported 
much work in the laboratory and tests in 
mines. They reported the relative radar 
transparency of various minerals and an- 
tenna design parameters in an actual mine 
test. Good results were obtained with a 
radar operating at 100 MHz with one-way 
propagation ranges from 6 to 56 ft and 
reflections through 28 ft of coal. These 
results were obtained despite having to 
use an inadequate antenna designed for 
the open air. Attenuation was from 1 to 
1.6 dB/ft of coal, and particularly good 
reflections were obtained from any target 
that was wet or contained water. Long 
wavelengths gave poor target resolution, 
but there was far less loss of energy by 
scattering if the impinging wave had a 
wavelength greater than either the thick- 
ness of the layer of coal, shale, or 
limestone, or the individual rock crystal 
size. The radar successfully detected 
mine openings through 30 to 60 ft of rock 
in narrow, cluttered, dirty, wet under- 
ground workings , and was unaffected by 
the presence of steel paneling or beams 
nearby. The outlook for deep electromag- 
netic probing in soft ground such as 
sand, earth, or mud was not so favorable. 
Even at low frequencies , 50 to 70 ft 
seemed to be the ultimate range. The ra- 
dar signals. Cook reported, were easy to 
interpret about half the time. Image in- 
terpretation was still in its infancy in 
underground radar, and there were many 
unexpected and unexplained results. 
There were spurious echoes on some occa- 
sions and no reflections on others. More 
work, it was suggested, needed to be done 
in mines under known conditions. The 
most important factor was the absorption 
of rock; less important were the choice 
of principal frequencies, the transmit- 
ter's electronic efficiency ("performance 
figure"), or the antenna's directivity. 
GPR, Cook said, offers great potential 
for exploring large volumes of rock at 



10 



low cost in advance of mining. Other 
tests (96) , of a road tunnel (with steel 
supports) in limestone, a fault in a do- 
lomite quarry, and a coal mine tunnel, 
seemed to bear out Cook's prediction. 

SHORT PULSE GPR 

Researchers at ENSCO, Inc., now XADAR 
Corp., developed a short pulse GPR, which 
was tested in coal mines in 1977 and 1978 
(39, 51 , 57) . The portable set consisted 
of a transmitter, a receiver, and a con- 
troller. The transmitter emits a short, 
high frequency (30- to 600-MHz) pulse 
that is coupled to the ground through the 
antenna. The transmitted pulse is shaped 
like a single cycle of a sine wave. The 
pulse is radiated from the antenna into 
the earth in a broad pattern. Whenever 
the radar pulse encounters a medium with 
different electrical properties, a por- 
tion of the electromagnetic energy is re- 
flected back to the receiver. The change 
in electrical properties coincides with 
changes in the nature of the mediiam, such 
as a fault or clay vein passing from the 
coal seam into shale or rock. By oieasur- 
ing the time of passage of the radar wave 
and its reflection, and by knowing the 
velocity of the wave, the distance to the 
anomaly can be determined. However, in 
coal and rock, unlike air, the speed of 
propagation is not a constant and must be 
separately determined for each site. It 
is normally in the range of 0.2 to 0.5 
times the speed of light. Using this 
type of GPR, ENSCO researchers were able 
to propagate (one way) a signal 100 ft in 
a coal seam, using signals in the 40- to 
500-MHz range. Voids were detectable us- 
ing the GPR at 50 ft, in the frequency 
range of 10 to 160 MHz. An uncased bore- 
hole was detected over 25 ft away. 

BOREHOLE RADAR 

Radar was also applied ( 119 , 145 ) to 
borehole survey. Under a Bureau con- 
tract. Southwest Research Institute ( 117 ) 
developed a borehole radar. The entire 
package was required to be less than 4 
in. in diameter and 1 m long, which re- 
quired a special antenna design to fit 



the cylindrical shape. Representative 
models of what the various geologic anom- 
alies in mines would look like on a radar 
return had to be developed. A short 
pulse, high-power radar, with a 15- to 
30-m range surrounding the borehole, was 
developed using a sampling oscilloscope 
and signal averaging technique. The data 
were digitized and fed into a computer 
for clutter rejection. An experimental 
system was built and tested in single- 
and cross-borehole operations. There 
were problems of data interpretation and 
with penetration range. Distant targets 
were dim because of medium attenuation 
and normal geometric dispersion, while 
near targets reflected very strongly. A 
time-varying gain amplifier was used to 
overcome this. The system proved more 
suitable for hard rock than for sedimen- 
tary rock, and there were problems of 
spurious signals from electric cables and 
other components. 

In a later phase of work, a borehole 
directional radar probe was developed by 
Southwest Research Institute ( 116 ) and 
subjected to demonstration testing in 
1982. The radar was a video pulse radar. 
Each pulse had a time duration of 12 ns 
and a useful frequency spectrum of 30 to 
300 MHz with a 1-kW peak power output. 
The probe (16.7 ft in length) was de- 
signed to fit within a 4-in borehole. 
The unit comprised the borehole probe and 
the surface control unit connected by or- 
dinary armored four-conductor wire line 
cable. The borehole probe transmitter 
utilized a unique rotating directional 
antenna. Earlier units tested with sin- 
gle antennas for transmitting and receiv- 
ing had proven unsatisfactory. The new 
antenna scanned in eight angular steps 
(45° apart) according to the command of 
the surface control unit. The actual or- 
ientation was determined by a built-in 
fluxgate north-seeker circuit for verti- 
cal drill holes or a pendulum vertical 
reference for horizontal holes and was 
reported to the surface unit. The re- 
ceiver antenna was a short, omnidirec- 
tional monopole. A time-domain sampler 
developed a low frequency replica of the 
reflected signal from the receiver output 



11 



by repeated sampling and enhancement by 
signal averaging. This replica was typi- 
cally in the audiofrequency range (3 to 
30 Hz or 300 to 3,000 Hz) and could be 
transmitted by the standard geophysical 
armored four-conductor wire line cable. 

The unit was field-tested in York Can- 
yon Mine near Raton, N. Mex. This mine 
was chosen for the tests because of ease 
of access and for its favorable electro- 
magnetic propagation characteristics, and 
because it had a well-documented geologic 
anomaly to serve as a target. The anom- 
aly was a 7-ft coal seam displaced 50 ft 
by a complex fault that had stopped min- 
ing development and had been encountered 
at points 700 ft apart. In the actual 
tests, the returns appeared to be omnidi- 
rectional, presumably because of the poor 
directional response of the antenna sys- 
tem. The fault was identified by one 
borehole probe as a target reflection 
from 22 ft away. Another borehole probe, 
20 ft from the fault, failed to get a 
clear response from the fault. 

As part of their work in developing 
cross-borehole methods ( 23 , 25 ) for use 
in detection of mining hazards for the 
Bureau of Mines , Lawrence Liveirmore Na- 
tional Laboratory (LLNL) undertook exper- 
iments at the Gold Hill Mine test site 
near Boulder, Colo., in 1980. The tests 
involved using a high frequency electro- 
magnetic (HFEM) system to locate a tunnel 
49.5 m below the surface between two 
boreholes 10 m apart. A transmitter, 
broadcasting at a constant frequency and 
amplitude at frequencies between 45 and 
70 MHz, was lowered down one borehole. 
The receiver was lowered down the other. 
Two experimental scenarios were used; in 
the first, the transmitter and receiver 
were lowered at a constant rate so that 
they were at the same or "common depth" 
at all times. In the second, the trans- 
mitter and receiver were lowered with a 
constant difference between their depths, 
which was maintained to provide "offset 
views" for better definition of the tar- 
get. Common-depth tests were run at 45, 
50, 55, 60, 65, and 70 MHz and offset- 
view tests at 45, 50, and 55 MHz. Data 



were plotted as signal strength versus 
depth, with the tunnel location indicated 
by signal minima. The tunnel was accu- 
rately located; it lay between test bore- 
holes only 10 m apart. The size of the 
tunnel was not so accurately determined; 
it was measured as having a height of 1.7 
m when it actually was 2.8 m high. 

Lawrence Livermore Laboratory also de- 
veloped and tested a high frequency (mi- 
crowave), cross-borehole electromagnetic 
system (81) designed to detect water- or 
air-filled underground cavities. At Man- 
atee Springs, Fla. , tests were run on 
known water-filled caverns using micro- 
wave propagation (5 to 105 MHz) between 
two boreholes , one containing the trans- 
mitting antenna and the other the receiv- 
ing. In this test, the test signal fre- 
quency was varied from 5 to 105 MHz and 
the depth from 30 to 38 m to encompass 
the area where the cavern was expected to 
be located. At Medford Caves, an air- 
filled cavern was mapped by using a 
single-frequency scan (100 MHz) and vary- 
ing the depth from 25 to 70 m. The tests 
indicated that air- or water-filled cavi- 
ties have characteristic electronic sig- 
natures but are difficult to identify 
if the cavity is irregularly shaped or 
if the matrix is extensively fractured. 
A medium of high conductivity creates 
problems also, by making it difficult 
to propagate a signal of sufficient 
strength. 

SYNTHETIC PULSE RADAR 

Under Bureau contract, XADAR Corp. 
(formerly ENSCO) is investigating a syn- 
thetic pulse radar for coal mine use 
(51). Short pulse systems have many dis- 
advantages: the need to separate the 
transmitted and reflected signal, sam- 
pling techniques that use only part of 
the transmitted energy, and complicated 
antenna design. A synthetic pulse radar 
system is designed to overcome noise 
problems, to give a higher signal-to- 
noise ratio, and to increase penetration 
using lower energy. The latter is neces- 
sary for permissibility for use in under- 
ground coal mines. The synthetic pulse 



12 



radar transmits a single frequency at a 
time. The amplitude and phase of the re- 
ceived signal are measured, and the fre- 
quency is then stepped to the next value. 
If the proper frequencies are transmitted 
and received, then a pulse can be recon- 
structed by taking the inverse Fourier 
transform of the received data. The sys- 
tem does require more signal and data 
processing and more sophisticated equip- 
ment. In the initial test, a breadboard 
unit was built and tested. It was de- 
signed for operation within a transmis- 
sion frequency range of 20 to 160 MHz 
with a minimum spacing of 100 kHz. This 
should give a maximum time "window" of 
10,000 ns or a reflection range of ap- 
proximately 2,500 ft in coal, much great- 
er than needed. In the breadboard tests, 
many problems with internal interference 
were encountered, and finally fiber op- 
tics had to be used to isolate some sys- 
tem elements. There were also problems 
with ghost signals. Nevertheless, it was 
shown that a prototype system having an 
effective penetration range of 200 ft 
could be developed. 

From the experience gained from the 
breadboard unit, a prototype synthetic 
pulse GPR was built (146). It was com- 
pleted in June 1982 and then tested. 
There were many changes from the original 
breadboard unit. The system used a het- 
erodyne receiver, which uses two mixers. 
This allowed the detection circuits to 
operate at the lower intermediate fre- 
quency (IF) of 1 MHz, which eliminated 
many problems associated with the higher 
frequency circuits. The signal sent to 
the control unit from the receiver was 
at the intermediate frequency. Despite 
equipment difficulties that prevented 
accomplishment of all the experimental 
objectives, the unit was successfully 
tested in Eastern and Western coal mines. 
It was found that the synthetic pulse GPR 
could penetrate greater distances (ap- 
proximately 2 times farther) than short 
pulse GPR. Reflection depths of 40 ft 
were achieved in the Deseret Mine, Utah, 
and the background noise was found to be 
60 dB lower than the main signal arrival. 



It was estimated, based upon these re- 
sults, that reflection ranges of 180 to 
200 ft are possible. Because of equip- 
ment difficulties, the antenna tuning 
feature planned for the unit was not 
built and the display unit was not fin- 
ished in time for the mine tests. More 
work on this equipment is planned. 

STRATA CONTROL RADAR 

The Bureau is also investigating a GPR 
for the identification of roof conditions 
in underground coal mines that might lead 
to roof falls. Most roof falls in coal 
mines are associated with geologic anom- 
alies within a few meters of the immedi- 
ate mine roof. Therefore, the primary 
interest of researchers is in very small, 
but hazardous rock features within a very 
narrow band of rock above or behind the 
surface of mine openings. This means 
that higher frequencies are required than 
for most GPR systems, in order to resolve 
the tiny anomalies, for example, cracks. 
Although attenuation is much greater for 
higher frequencies, the area of interest 
is of such shallow depth that the in- 
creased attenuation is an acceptable 
trade-off for better resolution. 

Primarily utilizing off-the-shelf com- 
ponents, a short pulse GPR was put to- 
gether in 1981, for tests and evaluation 
in the laboratory and mines. The trans- 
mitter of this radar emits a single pulse 
of 4-ns duration and 400-V amplitude at 
100 or 250 MHz. Both transmitter and re- 
ceiver utilize triangular dipole anten- 
nas. The receiver consists of a voltage 
variable attenuator, a high-gain pream- 
plifier, and a sampling unit. It is con- 
trolled by a small computer that performs 
several functions. The computer provides 
an external signal to the sampling unit 
to set the sampling gate on a particular 
point of the received pulse. It converts 
the sampling unit output to digital form 
and averages the signal from a number of 
pulses to remove noise. It then steps 
the sampling gate to the next point on 
the pulse and repeats the process. When 
a full pulse has been sampled, the 



13 



computer stores the data on a magnetic 
tape. In addition, it supplies an expo- 
nentially variable voltage to the voltage 
variable attenuator to adjust the receiv- 
er system sensitivity to compensate for 
the signal attenuation through the rock. 
At the completion of a run, the small 
computer can be connected to a large com- 
puter and the data transferred to a disk 
file for later analysis. 

Several surface and three underground 
tests of the strata control GPR were made 
in 1982 and 1983 in mines near Tusca- 
loosa, Ala. Figures 3 and 4 show the 



computer control console and antenna in- 
stallations underground. Radar signa- 
tures were correlated with rock strati- 
graphies. In tests, three rock strata 
were identified; the middleman rock 2 to 
3 ft thick, the overlying coal seam 3 to 
6 ft thick, and the main roof. These 
were correlated with ground-truthing test 
holes , drilled and borescoped in the 
mine roof. The data are currently being 
computer-enhanced so that they can be 
displayed in a two-dimensional graphic 
representation that reveals stratigraphic 
conditions and any anomalies encountered 
in the radar scan. 




FIGURE 3. - Radar control console of GPR used during underground testing. 



14 




FIGURE 4, - Antenna deployment against mine roofo 
GPR ANTENNAS 



A group from the Electromagnetics Divi- 
sion of NBS evaluated antennas suitable 
for GPR for the Bureau of Mines (8^). Af- 
ter a literature review had narrowed the 
possible choices, several antenna types 
were tested, most of which were horn-type 
antennas. Eventually one was selected as 
being the best compromise among competing 
requirements, for example, small physical 
size needed in mine work and some supe- 
rior electronic characteristics of larger 
antennas. Transverse electromagnetic 



(TEM) horns appeared to be the most prom- 
ising, so most of the tests were run on 
modifications of this basic type. Even- 
tually three antennas were built, which 
were TEM horns, labeled NBS-TG, They 
were small (49 cm long, 50 cm high, 21.5 
cm wide) , metal TEM horns with variable 
width and variable flare angle. The NBS 
group also put together two GPR systems , 
a pulse radar time-domain system and a 
FM-CW system. Only initial evaluations 
of these two systems were undertaken. 



INTERPRETATION OF GPR DATA 



The question then becomes not whether 
radar can penetrate the ground and be 
used to map ahead of mining, but increas- 
ingly one of analysis (9^, _22, 82^, 92, 97 , 
105 ) of radar returns to determine the 
nature of the target. For example, an 
ambitious study (23) was made by investi- 
gators at Ohio State whose purpose was 
to develop a simple, automatic, "single 
look," real-time method of identify- 
ing subsurface targets. A commercially 



available radar was used. It was a video 
pulse radar (direct current to more than 
3 GHz) with a very short pulse that al- 
lowed for "time isolation" or a radar 
"dead zone" between the transmitted and 
reflected signals. Separate orthogonal 
dipole antennas for transmission and re- 
ception aided signal separation. The or- 
thogonal antenna was Insensitive to lay- 
ering within the earth. The receiver 
used a sampling oscilloscope that reduced 



15 



signal noise by averaging a large num- 
ber of waveforms rather than by using 
filters. A signal processor was used 
for clutter and noise reduction, and 
another unit for target characterization 
and identification. A general purpose 
digital computer was used for characteri- 
zation and identification of the targets 
as well as for controlling the sampling 
oscilloscope. 

In the experiment, five targets of dif- 
fering shapes and materials (plastic 
landmine, brass cylinder, aluminum 
sphere, copper sheet, and woodboard) were 
buried at known positions and depths in a 
small yard at Ohio State University, Ra- 
dar reflection images of each target were 
obtained. The result was a processed 
time-domain waveform. These waveforms 
exhibited a transient behavior (a super- 
imposed signal upon the basic return 
waveform) in the time region late in the 
signal pulse, where only the natural re- 
sponse of the target had any effect. On 
analysis, the Fast Fourier Transforms 
(FFT) of these waveforms showed strong 
peaks indicative of resonance behavior. 
These proved unchanging with variations 
in antenna location or orientation. It 
was concluded from this that "the complex 
natural resonances of the target are ex- 
citation invariant." 

Thus, the individual targets produced 
"complex natural resonances," depending 
upon their shape and constituent mate- 
rial, which could be used as the basis 
for target characterization and identifi- 
cation. The signal level of these sever- 
al natural resonances varied greatly, 
creating a problem of dynamic range, and 
varying ground conditions introduced even 
more complications. The value of the 
complex natural resonances of the targets 
was calculated from experimental data by 



using Prony's method, a two-century-old 
system for the mathematical treatment of 
waveforms . These calculated waveforms 
were then compared to the actual wave- 
forms of the buried objects by the 
computer. 

The researchers at Ohio State continued 
to improve their process for using radar 
in identifying buried objects (25) . A 
redesigned antenna was a step forward. 
The new antenna resonated at the middle 
of the range of the band of complex reso- 
nances of the target. This provided a 
better match between the resonant fre- 
quency of the radar system and the fre- 
quency band spanned by the target reso- 
nances and allowed improved target 
identification. 

Interpreting the signal return re- 
flected back from a buried target is a 
difficult process. For targets in air, 
analytical methods modeled on physical 
optics are used to image the targets. 
The situation is more complex for buried 
targets, but target imaging is similarly 
based on the backscattered field ( 24 , 
148 ) . Investigators at Ohio State work- 
ing under an Army contract found that , 
for below-the-surface target imaging, the 
most important part of the signal is the 
early return (high frequency) part of the 
backscattered field. The backscattered 
field is attenuated by the ground, and 
this attenuation determines the signal 
strength and, therefore, the depth of 
penetration. The backscattered field 
contains several complex functions of the 
reflected radar pulse and is very diffi- 
cult to interpret. The problem of the 
interpretation of reflected subsurface 
radar signals remains one that will un- 
doubtedly require extensive attention in 
the future. 



CONCLUSIONS 



It has long been known that electromag- 
netic waves can penetrate earth and rock. 
Various GPR's in portable, low-powered 
forms have been used successfully to lo- 
cate subsurface objects and voids in mil- 
itary operations, highway construction. 



and archeological work at short ranges. 
This same technology could be used to de- 
tect hidden geologic anomalies in mine 
roof , ribs , or faces , which might pose a 
safety hazard to miners. Bureau of Mines 
research efforts have been directed 



16 



toward investigating various types of GPR 
systems capable of detecting such hidden 
hazards, ahead of and during mining. The 
electromagnetic radar systems that may 
have significance for exploration and 
mine safety include microwave radar. 



borehole radar, short pulse radar, syn- 
thetic pulse radar, and strata control 
radar. More research is needed to per- 
fect the systems that have been initi- 
ated, especially in the area of data 
analysis. 



BIBLIOGRAPHY 



1. Alongi, A. V. Pavement Thickness 
Measured, Voids Detected by Downward- 
Looking Radar In New York Test. Ind. 
Res. News, Fall 1973, pp. 36-39. 

2. Anderson, W. L. , and R. K. Moore. 
Frequency Spectra of Transient Electro- 
magnetic Pulses in a Conducting Medium. 
IRE Trans, Antennas and Propag. , v. AP-8, 
No. 4, 1960, pp. 603-607. 

3. Annan, A. P., and J. L. Davis. Im- 
pulse Radar Sounding in Permafrost. Ra- 
dio Sci., V. 11, No. 4, 1976, pp. 383- 
394. 

4. Arcone, S. A. Distortion of Model 
Subsurface Radar Pulses in Complex Di- 
electrics. Radio Sci., v. 16, No. 5, 
1981, pp. 855-864. 

5. Balanis, C. A., J. C. Jeffrey, and 
Y. K. Yoon. Electrical Properties of 
Eastern Bituminous Coal as a Function of 
Frequency, Polarization and Direction of 
the Electromagnetic Wave and Temperature 
of the Sample. Paper in Electromagnetic 
Guided Waves in Mine Environments. Pro- 
ceedings of a Workshop (Boulder, Colo. , 
Mar. 28-30, 1978), ed. by J. R. Wait 
(contract HOI 55008, Nat. Telecommun. and 
Inf. Administration). BuMines OFR 134- 
78, 1978, pp. 87-102; NTIS PB 289 742. 

6. Balanis, C. A. , W. S. Rice, and 
N. S. Smith. Microwave Measurements of 
Coal. Radio Sci., v. 11, No. 4, 1976, 
pp. 413-418. 

7. Balanis, C. A., P. W. Shepard, 
F. T. C. Ting, and W. F. Kardosh. Aniso- 
tropic Electrical Properties of Coal. 
IEEE Trans. Geosci. and Remote Sensing, 
V. GE-18, No. 3, 1980, pp. 250-256. 



8. Belsher, D. R. Detection of Lost 
Oil Well Casings and Unknown Water Filled 
Voids in Coal Mines Through Development 
of a Microwave Antenna System (contract 
H0272007, NBS). BuMines OFR 6-79, 1978, 
94 pp. 

9. Benson, R. C. Application of 
Ground Penetrating Radar to Geotechnical, 
Hydrological and Environmental Assess- 
ments. Abstracts, 48th Annual Meeting 
and Exposition, San Francisco, Calif., 
October 1978. Soc. Explor. Geophys., 
Littleton, Colo., 1978, p. 279. 

10. Benson, R. C, and R. A, Glaccum. 
Radar Surveys for Geotechnical Site As- 
sessment. Pres. at Am. Soc. Civil Eng. 
Conf., Geotechnical Engineering Div. , At- 
lanta, Ga. , October 1979, preprint, pp. 
1-16; available for consultation at Tus- 
caloosa Research Center, Bureau of Mines, 
Tuscaloosa, Ala. 

11. Bertram, C. L. , K. J. Campbell, 
and S. S. Sandler. Locating Large Masses 
of Ground Ice With an Impulse Radar Sys- 
tem. Proc. 8th Int. Symp. on Remote 
Sensing, Univ. Mich., Ann Arbor, Mich., 
October 1972, pp. 241-260. 

12. Bertram, C. L. , R. M. Morey, and 
S. S. Sandler. Feasibility Study for 
Rapid Evaluation of Airfield Pavements. 
U.S. Air Force Weapons Lab., Dayton, 
Ohio, AFWL-TR-7 1-178, June 1974, 38 pp. 

13. Bhattacharyya, B. K. Propagation 
of Transient Electromagnetic Waves in a 
Conducting Medium. Geophysics, v. 20, 
No. 4, 1955, pp. 959-961. 



17 



14. Bhattacharyya, B. K. Propagation 
of Transient Electromagnetic Waves in a 
Medium of Finite Conductivity. Geophys- 
ics, V. 22, No. 1, 1957, pp. 75-88. 

15. Biggs, A. W. Radiation Fields 
From a Horizontal Dipole in a Semi- 
Infinite Medium. IRE Trans. Antennas and 
Propag. , V. AP-10, No. 4, 1962, pp. 358- 
362. 

16. Biggs, A. W. , and H. M. Swarm. 
Radiation Fields of an Inclined Electric 
Dipole Immersed in a Semi-Infinite Con- 
ducting Mediinn. IEEE Trans. Antennas and 
Propag., V. AP-11, No. 3, 1963, pp. 306- 
310. 



24. Chan, L. C. , D. L. Moffatt, and 
L. Peters, Jr. Subsurface Radar Target 
Imaging Estimates. IEEE Trans. Anten- 
nas and Propag., v. AP-29, No. 2, 1981, 
pp. 413-417. 

25. Chan, L. C. , L. Peters, Jr., and 
D. L. Moffatt. Improved Performance of a 
Subsurface Radar Target Identification 
System Through Antenna Design. IEEE 
Trans. Antennas and Propag., v. AP-29, 
No. 2, 1981, pp. 307-311. 

26. Chew, C. , and J. A. Kong. Elec- 
tromagnetic Field of a Dipole on a Two- 
Layer Earth. Geophysics, v. 46, No. 3, 
1981, pp. 309-315. 



17. Burrell, G. A., and L. Peters, Jr. 
Pulse Propagation in Lossy Media Using 
the Low-Frequency Window for Video Pulse 
Radar Application. Proc. IEEE, v. 67, 
No. 7, 1979, pp. 981-990. 

18. Caffey, T. W. H. Two-Way Communi- 
cation With an Earth Penetrator. Radio 
Scl., V. 11, No. 4, 1976, pp. 267-273. 

19. Caldecott, R. Electromagnetic 
Pulse Sounding for Surveying Underground 
Water. Electroscience Lab. , Ohio State 
Univ., Columbus, Ohio, Rep. 401X-1, 1967, 
429 pp. 

20. Campbell, K. J. , and A. S. Orange. 
A Continuous Profile of Sea Ice and 
Freshwater Ice Thickness by Impulse Ra- 
dar. Polar Rec. , v. 26, 1974, pp. 31-41. 

21. Campbell, K. J., and J. Ulrichs. 
Electrical Properties of Rocks and 
Their Significance for Lunar Observation. 
J. Geophys. Res., v. 74, No. 25, 1969, 
pp. 5867-5881. 

22. Chan, L. C. A Digital Processor 
for Transient Subsurface Radar Target 
Identification. M.S. Thesis, Ohio State 
Univ., Columbus, Ohio, 1975, 141 pp. 

23. Chan, L. C. , D. L. Moffatt, and 
L. Peters, Jr. A Characterization of 
Subsurface Radar Targets. Proc. IEEE, v. 
67, No. 7, 1979, pp. 991-1000. 



27. Chlamtac, F., and F. Abramovici. 
The Electromagnetic Fields of a Horizon- 
tal Dipole Over a Vertically Inhomogene- 
ous and Anisotropic Earth. Geophysics, v. 
46, No. 6, 1981, pp. 904-915. 

28. Clay, C. S. , L. L. Greischor, and 
T. K. Kan. Matched Filter Detection of 
Electromagnetic Transient Reflections. 
Geophysics, v. 39, No. 5, 1974, pp. 683- 
691. 

29. Clough, J. W. Electromagnetic La- 
teral Waves Observed by Earth-Sounding 
Radars. Geophysics, v. 41, supplement, 
1976, pp. 1126-1128. 

30. Collett, L. S., and T. J. Katsube. 
Electrical Parameters of Rocks in Devel- 
oping Geophysical Techniques. Geophys- 
ics, V. 38, No. 1, 1973, pp. 76-91. 

31. Cook, J. C. An Electrical Cre- 
vasse Detector. Geophysics, v. 21, No. 
4, 1956, pp. 1055-1070. 

32. . Electrical Properties of 
Bituminous Coal Samples. Geophysics, v. 
35, No. 6, 1970, pp. 1079-1085. 

33. . Proposed Monocycle-Pulse, 

VHF Radar for Airborne Ice and Snow Mea- 
surements. Trans. AIEE, Commun. and 
Electron., v. 79, pt. 2, 1960, pp. 588- 
594. 



18 



34, Cook, J, C. Radar Exploration 

Through Rock in Advance of Mining, 

Trans, Soc, Min. Eng. AIME, v, 254, June 
1973, pp. 140-146, 



35. 



Radar Transparencies of 



Mine and Tunnel Rocks. Geophysics, v, 
40, No, 5, 1975, pp, 865-885. 



36. 

Radar, 



Seeing Through Rock With 



Ch, in N, Am, Conf. on Rapid Ex- 
cavation and Tunneling, Henniker, N,H, , 
1972. AIME, Chicago, 111., v. 1, 1972, 
pp. 89-101. 



37. 



Status of Ground-Probing 



Radar and Some Recent Experience. Proc. 
Eng. Foundation Conf., Subsurface Explo- 
ration for Underground Excavation and 
Heavy Construction, Henniker, N.H. , Aug. 
11-16, 1974. Am, Soc. Civil Eng., New 
York, 1974, pp. 175-194. 

38. . A Study of Radar Explora- 
tion of Coalbeds (Teledyne Geotech) . Bu- 
Mines OFR 5-72, 1971, 83 pp; NTIS PB PB 
207 362. 



Electro-Magnetic Sounder Experiment. 
Geophysics, v. 39, No. 1, 1974, pp. 49- 
55. 

44. Duckworth, K. Electromagnetic 
Depth Sounding Applied to Mining Prob- 
lems. Geophysics, v. 35, No. 6, 1970, 
pp. 1086-1098. 

45. Eberle, A. C, and J, D, Young. 
The Development and Field Testing of a 
New Locator for Buried Plastic and Metal- 
lic Utility Lines. Natl. Acad. Sci. — 
Natl. Res. Council, Washington, D.C., 
Transp. Res. Rec. 631, 1972, pp. 47-52. 

46. Ellerbruch, D. A. Coal Layer 
Thickness Measurement Using FM-CW Sys- 
tem in 1-2 GHz Band. IEEE Trans. 
Geosci. Electron., v. GE-16, No. 2, 1978, 
pp. 126-133. 

47. Ellerbruch, D. A. , and J. W. 
Adams . Microwave Measurement of Coal 
Layer Thickness (contract SO144086, NBS). 
BuMines OFR 101-75, 1974, 33 pp; NTIS COM 
74-11643. 



39. Coon, J, B, , J, C. Fowler, and 
C, J. Schafers, Experimental Uses of 
Short-Pulse Radar in Coal Seams. Geo- 
physics, V. 46, No. 8, 1981, pp. 1163- 
1168. 

40. Cooper, D. W. , R. A. Muller, and 
R. J. Schertler, Remote Profiling of 
Lake Ice Using an S-Band Short-Pulse Ra- 
dar Aboard and All-Terrain Vehicle. Ra- 
dio Sci., V. 11, No. 4, 1976, pp. 375- 
381. 

41. Curtis, W. L. , and R. J. Coe. A 
Microwave Technique for the Measurement 
of the Dielectric Properties of Soils. 
IEEE Trans. Microwave Theory and Tech. , 
V. MTT-11, No. 3, 1963, pp. 211-212. 

42. Dolphin, L. T. , W. B. Beatty, and 
J. D. Tunzi. Radar Probing of Victorio 
Peak, New Mexico. Geophysics, v. 43, No. 
7, 1978, pp. 1441-1448. 

43. Dolphin, L. T. , R. L. Bollen, 
and G. N, Oetzel. An Underground 



48. Ellerbruch, D, A,, and D, A, Bel- 
sher. Electromagnetic Technique of Mea- 
suring Coal Layer Thickness, IEEE Trans, 
Geosci, Electron,, v, GE-15, No. 2, 1977, 
pp. 126-133. 

49. . FM-CW Electromagnetic 

Technique of Measuring Coal Layer Thick- 
ness. NBS, NBSIR 76-840, May 1976, 67 pp. 

50. England, A. W. , and G. R. Johnson, 
Microwave Brightness Spectra of Layered 
Media, Geophysics, v, 42, No, 3, 1977, 
pp. 514-521. 

51. ENSCO, Inc. Mine Roof Stratigra- 
phy Using Electromagnetic Radar (contract 
J0366076). U.S. Dep. Energy, DOE FE- 
9141-1, June 1978, 221 pp. 

52. Evans, S. Progress Report on Ra- 
dio Echo Sounding, Polar Rec, v, 11, 
1967, pp. 413-420. 



19 



53. Evans, S. Radio Techniques for 
the Measurement of Ice Thickness. Polar 
Rec, V. 3, 1963, pp. 406-410. 

54. Fitz, D. C. Application of Remote 
Sensing in Hydrology. M.S. Thesis, Colo. 
State Univ., Ft. Collins, Colo., 1980, 
131 pp.; NTIS PB 81-111932. 

55. Forbes, L. M. (ed.). Radio Echo 
Exploration of the Antarctic Ice Sheet, 
1967. Polar Rec, v. 14, 1968, pp. 211- 
213. 

56. Fountain, L. S., F. X. Herzid, and 
T. E. Owen. Detection of Subsurface Cav- 
ities by Surface Remote Sensing Tech- 
niques, Fed. Highway Administration, 
Rep. FHWA-RD-75-8, June 1975, 81 pp.; 
NTIS PB 253-379. 

57. Fowler, J. C. Subsurface Reflec- 
tion Profiling Using Ground-Probing Ra- 
dar. Min, Eng. (Littleton, Colo.), v. 
33, No. 8, 1981, pp. 1266-1270. 

58. Fowler, J. C, S. D. Hale, and 
R. T. Houck. Coal Mine Hazard Detection 
Using Synthetic Pulse Radar (contract 
H0292025, ENSCO, Inc.). BuMines OFR 79- 
81, 1981, 86 pp.; NTIS PB 81-224412. 



Dissipation Factors for Six Rock Types 
Between 20 and 100 Megahertz. BuMines 
RI 6913, 1967, 21 pp. 

64. Hancock, J. C. , and P. A. Wintz. 
Signal Detection Theory. McGraw-Hill 
Book Co., New York, 1966, 247 pp. 

65. Harrison, C. H. Reconstruction of 
Subglacial Relief From Radio Echo Sound- 
ing Records. Geophysics, v. 35, No. 6, 
1970, pp. 1099-1115. 

66. Heacock, J. G. (ed.). The Struc- 
ture and Physical Properties of the 
Earth's Crust. Am. Geophys. Union, Wash- 
ington, D.C., 1971, 284 pp. 

67. Helstrom, C. W. Statistical The- 
ory of Signal Detection. Pergamon Press, 
Inc., New York, 1968, 470 pp. 

68. Hipp, J. E. Soil Electromagnetic 
Parameters as Functions of Frequency, 
Soil Density, and Soil Moisture. Proc. 
IEEE, V. 61, No. 1, 1974, pp. 98-102. 

69. Hluchanek, J. A. Radar Investiga- 
tion of the Hockley Salt Dome. M.S. The- 
sis, Texas A. & M. Univ., College Sta- 
tion, Tex., 1973, 149 pp. 



59. Frischknecht, F. C. Fields About 
an Oscillating Dipole. Q. Colo. Sch. 
Mines, v. 62, No. 1, 1967, p. 326. 

60. Ghose, R. N. Mutual Couplings 
Among Subsurface Antenna Array Elements. 
IEEE Trans. Antennas and Propag. , v. AP- 
11, No. 3, 1963, pp. 257-261. 

61. Glenn, W. E. , J. Ryu, S. H. Ward, 
W. J. Peeples, and L. L. Phillips. The 
Inversion of Vertical Magnetic Dipole 
Sounding Data. Geophysics, v. 38, No. 6, 
1973, pp. 1109-1129. 

62. Grant, F. S., and G. F. West. In- 
terpretation Theory in Applied Geophys- 
ics. McGraw-Hill Book Co., New York, 
1965, 584 pp. 

63. Griffin, R. E., and R. L. 
Marovelli. Dielectric Constants and 



70. Hoekstra, P., and A. Delaney. The 
Dielectric Properties of Soils at UHF and 
Microwave Frequencies. Program and Ab- 
stracts of Papers Presented at Meeting of 
URSI-Int. Union Radio Sci., Group on An- 
tennas and Propagation, Boulder, Colo., 
August 1973, pp. 112-130. 

71. Holser, W. T. , J. J. Brown, 0. A. 
Fredrickson, and R. R. Unterberger. Ra- 
dar Logging of a Salt Dome. Geophysics, 
v. 37, No. 7, 1972, pp. 889-906. 

72. Ilsley, L. C, H. B. Freeman, and 
D. N. Zellers. Experiments in Under- 
ground Communications Through Earth 
Strata. BuMines TP 433, 1928, 60 pp. 

73. Jakosky, J. J. Radio as a Method 
for Underground Communications in Mines. 
BuMines RI 2599, 1924, 4 pp. 



20 



74. Jakosky, J. J., and D. H. Zellers. 
Factors Retarding Transmission of Radio 
Signals Underground and Some Further Ex- 
periments and Conclusions. BuMines RI 
2651, 1924, 10 pp. 

75. Jesch, R. L. High Resolution 
Sensing Techniques for Slopability. Fed. 
Highway Administration, Rep. FHWA-RD-79- 
32, January 1979, 47 pp. 



84. Luchinonov, U. S. Radar Sensing 
of Mountain Glaciers. Sov. Phys. — Tech. 
Phys. (Engl. Transl.), v. 18, 1973, 
p. 415. 

85. Lytle, R. J. Measurement of Earth 
Medium Electrical Characteristics; Tech- 
niques, Results, and Applications. IEEE 
Trans. Geosci. Electron., v. GE-12, No. 
3, 1974, pp. 81-101. 



76. Joyce, J, W. Electromagnetic Ab- 
sorption by Rocks With Some Experimental 
Observations Taken at Mammoth Cave in 
Kentucky. BuMines TP 497, 1931, 28 pp. 



86. Lytle, R. J., and D. L. Lager. 

The Yosemite Experiments: HF Propagation 

Through Rock. Radio Sci. , v. 11, No. 4, 
1976, pp. 245-252. 



77. Kerr, J. L. Short Axial Length 
Broadband Horns. Abstracts, 22nd Ann. 
Symp., USAF Antenna Research and Devel- 
opment, Univ. 111., Oct. 11-13, 1972, 
pp. 14. 



87. Lytle, R. J., E. F. Laine, D. L. 
Lager, and D. T. Davis. Cross-Borehole 
Electromagnetic Probing to Locate High- 
Contrast Anomalies. Geophysics, v. 44, 
No. 4, 1979, pp. 1667-1676. 



78. Khalafalla, A. S. , and J. M. 
Viner. Rock Dielectrometry at Mega- and 
Giga-Hertz Frequencies. Program and Ab- 
stracts of Papers Presented at Meeting of 
URSI-Int. Union Radio Sci. , Group on An- 
tennas and Propagation, Boulder, Colo. , 
August 1973, pp. 146-163. 

79. Kraichmann, M. B. Handbook of 
Electromagnetic Propagation in Conducting 
Media. U.S. Dept. of the Navy, Document 
NAVMAT P. 2302, 1970, 1128 pp. 



88. Lytle, R. J., E. F. Laine, D. L. 
Lager, and J. T. Okada, The Lisbourne 
Experiments: HF Propagation Through Per- 
mafrost Rock. Lawrence Livermore Lab. , 
Livermore, Calif., Rep. UCRL-51474, 1973, 
147 pp. 

89. Lytle, R. J., E. K. Miller, and 
D. L. Lager. A Physical Explanation of 
Electromagnetic Surface Wave Formulas. 
Radio Sci., v. 11, No. 4, 1976, pp. 235- 
243. 



80. Kuo, J. T. , and Dong-heng Cho. 
Transient Time-Domain Electromagnetics. 
Geophysics, v. 45, No. 2, 1980, pp. 271- 
291. 

81. Laine, E. F. Detection of Water- 
Filled and Air-Filled Underground Cavi- 
ties (BuMines contract H0202007, Lawrence 
Livermore Lab.). Lawrence Livermore 
Lab., Livermore, Calif., Rep. UCRL-53127, 
December 1980, 15 pp. 

82. Lee, K. H. , D. F. Pridmore, and 
H. F. Morrison. A Hybrid Three- 
Dimensional Electromagnetic Modeling 
Scheme. Geophysics, v. 46, No. 6, 1981, 
pp. 796-805. 



91. McGehan, F. P. Radar to the Res- 
cue. Dimensions/NBS, v. 63, No. 9, 1979, 
pp. 5-9. 

90. Meakins , B. J. Mechanisms of Di- 
electric Absorption in Solids. Ch. in 
Progress in Dielectrics-3. John Wiley & 
Sons, Inc., New York, 1961, 486 pp. 

92. Mofatt, D. L. Electromagnetic 
Pulse Sounding for Geological Surveying 
With Application in Rock Mechanics and 
Rapid Excavation Program (BuMines/ARPA 
(DOD) contract H02 10042, Ohio State 
Univ.). Electroscience Lab., Ohio State 
Univ., Columbus, Ohio, Semiannual Tech, 
Rep. 3190-1, October 1971, 79 pp. 



83. Lerner, R. M. Ground Radar Sys- 
tem. U.S. Pat. 3,831,173, Aug. 20, 1974. 



21 



93. Mofatt, D. L. Subsurface Appli- 
cation of Periodic Electromagnetic Video 
Pulse Signal, Paper in Thru-the-Earth 
Electromagnetic Workshop (Golden, Colo. , 
December 1973) , ed. by R. G. Geyer (con- 
tract GOl 33023, Colo. Sch. Mines). Bu- 
Mines OFR 16-74, 1974, pp. 112-117; NTIS 
PB 231 154. 



94. 



Subsurface Video Pulse 



Radar. Proc. Eng. Foundation Conf., Sub- 
surface Exploration for Underground Exca- 
vation and Heavy Construction, Henniker, 
N.H. , Aug. 11-16, 1974. Am. Soc. Civil 
Eng., New York, 1974, pp. 195-212. 

95. Moffatt, D. L. , and R. K. Mains. 
Detection and Discrimination of Radar 
Targets. IEEE Trans. Antennas and Prop- 
ag., V. AP-23, No. 3, 1975, pp. 358-367. 



102. Page, D. F., G. 0. Venier, and 
F. R. Cross. Snow and Ice Depth Measure- 
ments by High Range Resolution Radar. 
Can. Aeronaut, and Space J., v. 19, No. 
10, 1973, pp. 531-533. 

103. Parkhomenko, E. I. Electrical 
Properties of Rocks. Plenum Press, New 
York, 1967, 314 pp. 

104. Peters, L. , Jr. Electromagnetic 
Transient Underground Radar (ETUR) for 
Geophysical Exploration. Proc. IEEE An- 
tennas and Propag. Soc. Int. Symp., Am- 
herst, Mass., Oct. 10-15, 1976, APSIS 
786, pp. 203-204. 

105. Porcello, L. J. The Apollo Lunar 
Sounder Radar System. Proc. IEEE, v. 62, 
No. 6, 1974, pp. 769-783. 



96. Moffatt, D. L. , and R. J. Puskar. 
A Subsurface Electromagnetic Pulse Radar. 
Geophysics, v. 41, No. 3, 1976, pp. 506- 
518. 

97. Moore, D. F., and E. A. Quincy. 
Bayes Classification of Subsurface Elec- 
tromagnetic Responses. Radio Sci. , v. 
11, No. 4, 1976, pp. 395-403. 

98. Moore, J. R. , J. D. Echard, and 
C. G. Neill. Radar Detection of Voids 
Under Concrete Highways. Proc. IEEE Int. 
Radar Conf., Washington, D.C., April 28- 
30, 1980, pp. 130-135. 

99. Morey, R. M. Application of 
Downward Looking Impulse Radar. Proc. 
13th Annu. Can. Hydrographic Conf. , Cana- 
da Centre for Inland Waters, Burlington, 
Ontario, March 1974, pp. 83-99. 



100. 



Continuous 



Subsurface 



Profiling by Impulse Radar. Proc. Eng. 
Foundation Conf., Subsurface Exploration 
for Underground Excavation and Heavy Con- 
struction, Henniker, N.H. , Aug. 11-16, 
1974. Am. Soc. Civil Eng., New York, 
1974, pp. 212-232. 

101. Morey, R. M. , and W. S. Harring- 
ton, Jr. Feasibility Study of Electro- 
magnetic Subsurface Profiling. U.S. EPA, 
EPA R2-72-082, October 1972, 28 pp. 



106. Robin, G. de Q. , S. Evans, and 
J. T. Bailey. Interpretation of Radio 
Echo Sounding in Polar Ice Sheets. 
Philos. Trans. R. Soc. London, Ser. A, v. 
265, 1969, pp. 437-505. 

107. Robinson, L. A., W. B. Weir, and 
L. Young, Location and Recognition of 
Discontinuities in Dielectric Media Using 
Synthetic RF Pulses. Proc. IEEE, v. 62, 
No. 1, 1974, pp. 36-43. 

108. Rollwitz, W, L, , and J, D, King. 
Coal Thickness Gauge Using RRAS Tech- 
niques. (Natl. Acad. Sci. contract NAS 
8-32606, Southwest Res. Inst.). Final 
Rep., pt. 1, SWRI Project 15-4967, June 
10, 1978, 55 pp. 

109. Rubin, L. A., and J. C. Fowler. 
Ground Probing Radar for Delineation of 
Rock Features. Eng. Geol. (Amsterdam), 
V. 12, No. 2, 1978, pp. 163-170. 

110. Saint-Amant, M. , and D. W. 
Strangway. Dielectric Properties of Dry 
Geologic Materials. Geophysics, v. 35, 
No. 4, 1970, pp. 624-645. 

111. Sinha, A. K. , and P. K. Bhatta- 
charya. Vertical Magnetic Dipole Buried 
Inside a Homogenous Earth. Radio Sci., 
V. 1, No. 3, 1966, pp. 379-395. 



22 



112. Slichter, L. B. 
and Theoretical Response 
Ing Spheres. Trans. AIME, 
pp. 443-459. 



The Observed 

of Conduct- 

V. 97, 1932, 



121. Unterberger, R. R. Looking 
Through Rock. With Radar. Mln. Congr. J., 
V. 63, No. 6, 1977, pp. 38-41. 



113. Smith, B. D. , and S. H. Ward. On 
the Computation of Polarization Ellipse 
Parameters. Geophysics, v. 39, No. 6, 
1974, pp. 867-869. 

114. Stewart, R. D. Radar Investiga- 
tion of the Cote Blanche Salt Dome. M.S. 
Thesis, Texas A. & M. Univ., College Sta- 
tion, Tex., 1974, 347 pp. 

115. Stewart, R. D. , and R. R. Unter- 
berger. Seeing Through Rock Salt With 
Radar. Geophysics, v. 41, No. 1, 1976, 
pp. 123-132. 

116. Suhler, S. A., and T. E. Owen. 
Field Demonstration of Deep-Penetrating 
Geophysical Techniques (contract 
H0212018, Southwest Res. Inst.). Draft 
final rep., January 1983, 59 pp.; avail- 
able from Denver Research Center, U.S. 
Bureau of Mines, Denver, Colo. 

117. Suhler, S. A., T. E. Owen, J. E. 
Hipp, and W. R. Peters. Development of 
a Deep Penetrating Borehole Geophysical 
Technique for Predicting Hazards Ahead of 
Coal Mining (contract H0252033, Southwest 
Res. Inst.). BuMlnes OFR 77-80, 1978, 
124 pp.; NTIS PB 80-208614. 

118. Tarn, K. Dielectric Property Mea- 
surements of Rocks In the VHF-UHF Region. 
Ph.D. Thesis, Texas A. & M. Univ., Col- 
lege Station, Tex., 1974, 416 pp. 

119. Tarantolo, P. J., Jr., and R. R. 
Unterberger. Radar Detection of Bore- 
holes In Advance of Mining (Rock Salt). 
Geophys. Prospect., v. 26, No. 2, 1978, 
pp. 359-382. 

120. Trompeter, J. Where No Man Has 
Gone Before: A Report of Microwave Tech- 
niques In Subsurface Investigation. Con- 
str. Specifier, November 1980, pp. 38-47. 



122. 

Salt. Geophys. Prospect., 



Radar Propagation In Rock 
V. 26, No. 2, 



1978, pp. 312-328. 

123. U.S. Bureau of Mines. Annual Re- 
port of the Director of the Bureau of 
Mines to the Secretary of the Interior, 
1924, p. 16. 

124. Von Hlppel, A. R. Dielectrics 
and Waves. MIT Press, Cambridge, Mass., 
1954, 284 pp. 

125. Walt, J. R. A Conducting Perme- 
able Sphere In the Presence of a Coll 
Carrying an Oscillating Current. Can. J, 
Phys., V. 31, No. 4, 1953, pp. 670-678. 

126. . A Conducting Sphere In a 

Time-Varying Magnetic Field. Geophysics, 
V. 16, No. 4, 1951, pp. 666-672. 

127. . Electromagnetic Waves in 

Stratified Matter. Pergamon Press, Inc., 
Oxford, England, 1962, 372 pp. 



128. 

Ing Magnetic 



Induction by an Osclllat- 

Dlpole Over a Two-Layer 

Appl. Scl. Res., Sec. B, v. 7, 



Ground. 

1959, pp. 73-80. 

129. . Mutual Coupling of Loops 

Over a Homogeneous Ground. Geophysics, 
V. 20, No. 3, 1955, pp. 630-637. 

130. . Mutual Coupling of Wire 

Loops Lying on a Homogeneous Ground. 

Geophysics, v. 19, No. 2, 1954, pp. 290- 
296. 

131. . Mutual Electromagnetic 
Coupling of Loops Over a Homogeneous 
Ground-An Additional Note. Geophysics, 
V. 21, No. 2, 1956, pp. 479-484. 



23 



132. Walt, J. R. Note on the Theory 
of Transmission of Electromagnetic Waves 
in a Coal Seam. Radio Sci., v. 11, No. 
A, 1976, pp. 263-265. 



133. 



On 



Anomalous Propagation 
of Radio Waves in Earth Strata. Geophys- 
ics, V. 19, No. 2, 1954, pp. 342-343. 



134. 



Propagation of Electro- 



magnetic Pulses in a Homogeneous Conduct- 
ing Earth. Appl, Sci. Res., Sec. B. , v. 
8, No. 7, 1960, pp. 213-253. 



135. 



The Radiation Fields of 



a Horizontal Dipole in a Semi-Infinite 
Dissipative Medium. J. Appl. Phys., v. 
24, No. 3, 1953, pp. 958-959. 



136. 



Radiation From a Small 



Loop Immersed in a Semi-Infinite Conduct- 
ing Medium. Can. J. Phys., v. 37, No. 3, 
1959, 672-674. 



137. 



Transmission and Reflec- 



tion of Electromagnetic Waves in the 
Presence of Stratified Media. J. Res. 
NBS, V. 61, September 1958, pp. 205-232. 

138. Wait, J. R. (ed.). Electromag- 
netic Probing in Geophysics. The Golem 
Press, Boulder, Colo., 1971, 391 pp. 

139. Wait, J. R. , D. Chang, R. J. Ly- 
tle, M. A. Ralston, K. R. Umashankar, and 
R. C. Wittman. Analyticas Bases for 
Electromagnetic Sensing of Coal Proper- 
ties. U.S. Dep. Energy, Rep. DOE FF 
8972-1, July 1978, 34 pp. 

140. Ward, S. H. Electrical, Elec- 
tromagnetic and Magnetotelluric Meth- 
ods. Geophysics, v. 45, No. 11, 1980, 
pp. 1659-1660. 



141. Ward, S. H. Electromagnetic The- 
ory for Geophysical Applications. Min. 
Geophys., v. 2, 1967, pp. 10-197. 

142. Ward, S. H. , and D. C. Fraser. 
Conduction of Electricity in Rocks. Ch. 
in Mining Geophysics, Soc. of Explor. 
Geophys., Tulsa, Okla. , v. 2, 1970, 36 
pp. 

143. Ward, S. H. , and H. F. Morrison 
(eds.). Special Issue on Electromagnetic 
Scattering. Geophysics, v. 36, No. 1, 
1971, pp. 1-258. 

144. Watt, A. D. , F. S. Mathews, and 
E. L. Maxwell. Some Electrical Charac- 
teristics of the Earth's Crust, Proc. 
IEEE, v. 51, No. 6, 1963, pp. 897-910. 

145. Wu, T. T., and R. W. P. King. 
The Cylindrical Antenna With Nonreflect- 
ing Resistive Loading. IEEE Trans. An- 
tennas and Propag. , v. AP-13, No. 5, 
1965, pp. 369-373. 

146. XADAR Corp. Coal Mine Hazard De- 
tecting Using Synthetic Pulse Radar (con- 
tract H0212016). Draft final rep., De- 
cember 1982, 71 pp.; available from 
Denver Research Center, U.S. Bureau of 
Mines, Denver, Colo. 

147. Yatsyshin, V. I., N. I. Zhuk, K. 
M. Salamatov, and G. E. Yakovitskaya. 
Methods of Measuring the True and Effec- 
tive Electrical Conductivities of Rocks 
in Coal Mines. Sov. Min. Sci. (Engl. 
Transl.), v. 7, No. 7, 1971, pp. 453-456. 

148. Young, J. D. Radar Imaging From 
Ramp Response Signatures. IEEE Trans. 
Antennas and Propag., v. AP-24, No. 3, 
May 1976, pp. 276-282. 



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